I would absolutely calibrate, it's a good habit to get into from time to time, as the accelerometer and gyro can get bumped around in transit and with rough landings, etc. First though, I would inspect your props, make sure there are no significant dings, nicks or warping in them, that can cause some control issues. Next I would make sure your trims (the little tabs around your sticks) are set to "0" and test the copter there. They make a different beep for the center and end points, so play around with them until you find the one in the middle. Try launching the copter from there, and see how it goes. If that doesn't work, it's time to calibrate. The H100 runs a Multi Wii board, and the manual can be downloaded from the product page on Xheli. The manual details how you can calibrate either with the GUI (connecting to computer) or with commands from the transmitter (Tx). If you hold the throttle all the way down, bring the directional control all the way down, and then slide the throttle over to bottom left, that will start the gyro calibration. You'll notice an LED on the board start flashing, once it's done, it will lock solid. After that, the ACC can be calibrated by holding the throttle down, bringing the directional control down, and then moving the throttle to top left. Again, you'll see the LED flashing, and when it's done, it will lock solid. Now that everything is in tune, give it another shot. If you're still having some drift issues, try adjusting your trims to compensate. Let us know how it goes.